OCT is commonly used to image tissue for ophthalmic analysis, such as retinal imaging and analysis. OCT is also used to image skin tissue and OCT has been explored as a technique for measuring glucose concentration. For example U.S. Pat. No. 6,725,073 by Motamedi, et al., titled “Methods for noninvasive analyte sensing” describes using OCT to measure glucose concentration.
An OCT system, such as the polarized multiple reference system OCT system, consistent with prior art, is depicted in FIG. 1. A broadband optical source, such as a super-luminescent diode (SLD) 101, is fiber coupled through a length of optical fiber 123 with a fiber collimator (not shown) and emits a collimated optical beam 102 which is transmitted through an optional polarizer 103 through a half-wave plate 104 and split by a polarized beam-splitter 105 into reference radiation 106 and probe radiation 112. The length of optical fiber 123 acts as a spatial filter to filter the optical radiation and thereby improve the quality of interference signals.
The reference radiation 106 is transmitted through an attenuator 107 and a quarter wave plate 108 and then partially through a partial reflective mirror 109 to a reference mirror 110 mounted on a oscillating translation device 111, such as a voice coil or piezo device. The combination of the partial mirror 109 and the reference mirror 110 generates multiple reference signals as described in the patents incorporated herein by reference.
As the reflected reference radiation is transmitted back through the quarter wave plate 108, its polarization vector is rotated such that it will be re-directed by the polarized beam-splitter 105 towards the detection system depicted in the dashed box of FIG. 1.
The probe radiation 112 is transmitted through a second quarter wave plate 113 and through an anti-reflection coated blank that compensates for effects of the optical elements in the reference path. The probe radiation 112 is scattered by components in the target 115. Some of the probe radiation is scattered back through the quarter wave plate 113 where the double pass through the quarter wave plate 113 rotates its polarization vector by ninety degrees thereby enabling this scattered probe radiation to be transmitted through the polarized beam-splitter 105 towards the detection system.
The combined scattered probe radiation and reflected reference radiation is transmitted through an optional second half wave plate 116 to a second polarized beam splitter 117 that reflects one set of components of the reflected reference and scattered probe radiation to a detector 118 and transmits the orthogonal set of components of the reflected reference and scattered probe radiation to a detector 119 thereby achieving balanced detection.
In some embodiments the optional second half wave plate 116 is not present but the second polarized beam splitter 117 is rotated forty five degrees about the optical beam 120 so that again the polarized beam splitter 117 reflects one set of components of the reflected reference and scattered probe radiation to a detector 118 and transmits the orthogonal set of components of the reflected reference and scattered probe radiation to a detector 119.
Operation of the OCT system is controlled by means of a control module 121. The detected signals are processed by a processing module 122 to yield imaging and analysis of the target. The OCT system typically includes one or more lens to focus the reference and probe radiation. In the embodiment depicted in FIG. 1 a single lens 124 focuses both the reference and the probe radiation. Such systems also typically include one or more detector lenses.
In the embodiment depicted, the broadband optical source, such as a super-luminescent diode (SLD) and lens combination 101, that emits the collimated optical beam 102 includes a fiber that couples the SLD, via a fiber collimator that delivers the collimated beam to the rest of the optical system. In such fiber coupled systems the fiber acts as a spatial filter that delivers a high quality collimated beam. In fiber based OCT systems, (in contrast to free space OCT systems) the beam splitter function is also typically accomplished in fiber and there is therefore extensive fiber based spatial filtering.
However, in free space based OCT systems without fiber coupling between the SLD source and the collimated beam, the lack of spatial filtering degrades the performance of the OCT system. Approaches to accomplish spatial filtering by focusing the output of the SLD through a pin-hole, in addition to adding complexity, have the undesirable consequence of reflecting light back to the SLD which can cause unacceptable noise related problems.
An optical isolator, typically using polarization components, can be installed between the SLD source and the pin-hole to substantially reduce undesirable light reflected back from the pin-hole to the SLD. However, this approach requires additional optical components with associated cost and complexity.
While fiber based OCT systems have the advantage of fiber spatial filtering of the optical beam, free space OCT systems have the significant advantage of being potentially very low cost and thereby offering significant commercial advantage. There is therefore an unmet need for a free space OCT system that has spatial filtering without the undesirable reflections back to the SLD and without the need for additional optical components.